Caffeine is a stimulant of the central nervous system that occurs naturally in over 60 species of plants and is used in several foods, drinks and medications. The United States Food and Drugs Administration classes caffeine as both a food additive and a drug.

Pharmacokinetics

The half-life of caffeine (time taken for the body to eliminate one-half of the caffeine) varies widely between people, depending on factors such as age, body weight, pregnancy status, medication intake and liver health. In healthy adults, the half-life is approximately 5 to 6 hours. Heavy cigarette smoking can decrease the half-life of caffeine by up to a half and in pregnancy, the half-life may be increased by as much as 15 hours.

Caffeine is processed or metabolized in the liver by the cytochrome P450 oxidase enzyme system and broken down into three metabolic dimethylxanthines. These include:

1. Paraxanthine (forms 84%), which breaks down fats and increases blood levels of glycerol and fats.
2. Theobromine (forms 12%), which dilates blood vessels and also has a diuretic effect, increasing urination.
3. Theophylline (forms 4%), which dilates the airways and is used in the treatment of asthma.

These metabolites are then further broken down and excreted in the urine.

Caffeine crosses the blood-brain-barrier, which is designed to separate the brain from the bloodstream. Once inside the brain, caffeine blocks the effects of adenosine, which plays an important part in energy transfer and sleep promotion. 

Caffeine’s effects on the body

The stimulatory effects of caffeine may begin as early as 15 minutes after ingesting the drug and last as long as six hours. In moderate doses, caffeine helps to increase alertness and reduces sleepiness. However, regular ingestion of excess amounts of caffeine can lead to problems such as poor concentration, nervousness, heartburn, constipation and diarrhea. Longer-term effects may include sleep deprivation, impaired judgement, emotional fatigue, mood swings, depression and anxiety.

Symptoms of excess caffeine use

Some examples of the symptoms that may occur in people who consume too much caffeine include:

• Anxiety
• Confusion
• Irritability
• Suppressed appetite
• Sleeplessness
• Dizziness
• Blurred vision
• Dry mouth
• Increased thirst
• Flushed skin
• Cold sweats
• Pale and clammy appearance
• Rapid heart rate and palpitation
• High blood sugar
• Breathing difficulty
• Tremors
• Heartburn
• Stomach ache
• Nausea and vomiting
• Diarrhea
• Increased urination
• Presence of ketones in the urine

Withdrawal symptoms

If stopped abruptly, caffeine may also give rise to withdrawal symptoms. Examples of these include:

• Irritability
• Headaches
• Loss of concentration
• Sleepiness in presence of insomnia
• Stomach pain

These effects may appear within 12 to 24 hours after stopping caffeine intake and last for 5 to 7 days. Analgesics such as aspirin can help to relieve these symptoms.

Caffeine uses

Some examples of the benefits people experience with caffeine intake include:

• Overcoming sleep deprivation – Caffeine can increase alertness and reduce sleepiness
• Mental acuity – Caffeine is though to boost concentration and working memory
• Physical performance – Caffeine can enhance physical performance, reducing perception of muscle pain and increasing energy.
• Headache reliever – Blood vessels often dilate during a headache and as a vasoconstrictor, caffeine is thought to be useful for easing headaches.
• Studies have demonstrated that the regular, moderate consumption of caffeine may prevent gallstones.
• Caffeine may also play a preventative role in the development of Parkinson’s disease and Alzheimer’s disease.

When to avoid caffeine

Caffeine intake should be avoided by pregnant women. Excessive caffeine intake during pregnancy has been linked to low birth weight, premature delivery and miscarriage. The FDA advises that pregnant women should avoid or strictly limit caffeine-containing foods and drugs.

Caffeine intake should also be avoided in the following health conditions:

• Sleep disorders
• High blood pressure
• Liver or kidney disease
• Anxiety or depression
• Gastroesophageal reflux disease

Further Reading

• All Caffeine Content
• What is Caffeine?
• What is Decaffeination?
• Caffeine Occurrence
• Caffeine consumption: How much is too much?

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.